Improved Circulating Current Control Method to Reduce Energy Storage and Current Rating Requirements in MMC

Modular multilevel converter (MMC) is the most competitive topology for high and medium-voltage applications. It is known that any voltage mismatch between its phase legs causes a second-harmonic-dominant circulating current (CC) to flow in each of the MMC arms. This current, if not controlled properly, leads to reduced efficiency, larger energy storage requirements, and increased current rating of semiconductor switches. To address all these issues, a new CC control scheme is proposed in this paper, which is based on the principle of injecting a second-harmonic circulating current (SHCC) with a predefined magnitude and phase angle. A design methodology is presented to determine the desired magnitude and phase angle of the SHCC. In addition, a comparative study involving the proposed and other major CC control schemes is also presented to establish the main features of the proposed principles. Using the proposed control scheme, the overall energy storage requirement can be reduced by 30% without increasing the current rating of semiconductor switches. The efficacy of the proposed scheme is verified and comparatively evaluated using a 13-level, grid-connected simulation model and also using a 7-level, grid-connected MMC experimental setup.